Method for forming plating film

ABSTRACT

A method for forming a plating film, comprising the steps of: applying a plating film onto an object to be plated at a first current density for a predetermined period in a plating bath having a cathode capable of varying current and an anode and; and maintaining the object to be plated at a second current density lower than the first current density. According to the present invention, it is possible to improve solderability of a plating film for conventional lead-free solder by a simple method, which allows the productivity to further enhanced, resulting in a plating film with reduced production costs.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a division of application Ser. No. 10/178,304, filedJun. 25, 2002, now U.S. Pat. No. 6,811,672, the entire content of whichis hereby incorporated by reference in this application.

This application is related to Japanese applications Nos. 2001-194740and 2002-054336, filed on 27 Jun., 2001 and 28 Feb., 2002 whosepriorities are claimed under 35 USC §119, the disclosures of which areincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for forming a plating film andto an electronic component having a plating film formed thereon by thesame method. More particularly, the present invention relates to amethod for forming a plating film which is lead-free and excellent insolderability and to an electronic component having a plating filmformed thereon by the same method.

2. Description of Related Art

In the field of the electric and electronic industries, where solderingis generally used for formation of circuits, an Sn—Pb, containing alloyis typically used as a soldering material and for rapid and ensuredsoldering, a plating film of the Sn—Pb-containing alloy is applied ontoan object to be soldered before hand.

For example, as shown in FIG. 7, a photo coupled semiconductor device40, which utilizes soldering, is constituted as follows. A lightemitting element 42 and a photoreceptor 43 are connected to headers oflead frames 41, respectively, by die bonding. The lead frames 41 aremade of a metal such as a Cu alloy or Fe alloy and are bent in advance.The light emitting element 42 and the photoreceptor 43 are connected tothe adjacent lead frames 41, respectively, by wire bonding using goldwires 48. The light emitting element 42 is precoated with resin 44. Thelead frames 41 having the light emitting element 42 and thephotoreceptor 43 connected thereto by die bonding are positioned andopposed to each other by spot welding or using a loading frame set, andan inner package is formed by primary transfer molding using a lighttransmitting epoxy resin 45. Further, an outer package is formed bysecondary transfer molding using a light shielding epoxy resin 46, and anon-glossy (non-shiny or lusterless) SnPb plating film is applied ontoportions of the lead frames 41 extending outside the outer package.

Thus, SnPb, which is inexpensive, has a low melting point, and isexcellent in processibility, has been used as a material for solderingand also as a material for a plating film. In recent years, however, itbecomes urgent to replace SnPb by a material which does not contain lead(lead-free material), as an environmental protection measure. Examplesof such a material for soldering include an alloy containing tin as aprincipal component together with one or more selected from silver,bismuth, copper, zinc and the like, and examples of such a material fora plating film include an alloy containing tin as a principal componenttogether with one or more selected from silver, bismuth, copper, zincand the like; gold; silver; palladium; and the like.

Japanese Unexamined Patent Publication No. 2000-26994 proposes an SnCuplating coat layer whose cryatal on a surface is fined so that thesurface is in a gloss state or a half gloss state.

However, it has been found that a plating coat layer as mentioned abovehas a zero cross time of 5 seconds or more, indicating that the coatlayer is poor in solderability and appearance when the solderability ofthe plating coat layer is evaluated by a meniscograph method byimmersing a component which has the coat layer as mentioned above into asoldering bath filled with a soldering alloy comprising Sn—Ag (3.5%) at270° C., at a predetermined rate and to a predetermined depth for fiveminutes and then picking it upward.

BRIEF SUMMARY

The present invention provides a method for forming a plating film,comprising the steps of: applying a plating film onto an object to beplated at a first density for a predetermined period in a plating bathhaving a cathode and an anode and capable of varying current andmaintaining the object to be plated at a second current density lowerthan the first current density or immersing the object to be plated intoa solution containing 5 to 10% wt of trisodium phosphate.

Further, the present invention provides an electronic component having aplating film formed thereon by the above method.

According to the present invention, it is possible to improvesolderability of a plating film for conventional lead-free solder by asimple method, which allows the productivity to further enhanced,resulting in a plating film with reduced production costs.

These and other objects of the present application will become morereadily apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing an embodiment of a method for forming aplating film in accordance with the present invention;

FIG. 2 is a schematic cross sectional view of an essential part forshowing an arrangement of anodes, a cathode and an object to be platedin the method for forming a plating film in accordance with the presentinvention;

FIG. 3 is a cross sectional view of a plating film formed by the methodfor forming a plating film in accordance with the present invention;

FIG. 4 is a cross sectional view of a plating film for comparison withthat in FIG. 3;

FIGS. 5(a) and (b) are schematic cross sectional views of an essentialpart for showing an arrangement of anodes, cathodes and objects to beplated in the method for forming a plating film in accordance with thepresent invention;

FIGS. 6(a) and (b) are schematic cross sectional views of an essentialpart for showing an arrangement of an anode, a cathode and objects to beplated in the method for forming a plating film in accordance with thepresent invention;

FIG. 7 is a schematic cross sectional view of an electronic componenthaving a plating film formed thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method for forming a plating film in accordance with the presentinvention includes the step of applying a plating film onto an object tobe plated at a first current density for a predetermined period in aplating bath having a cathode capable of varying current and an anode.

The object to be plated in accordance with the present invention is notparticularly limited, and may be any member as long as it is formed of aconductive material and may be made of any material, size and shape.Examples are an electronic device and a passive element such as an ICsemiconductor device, a photo coupled semiconductor device, a lightemitting element, a photoreceptor, a solid-state relay, a regulator, aresistor, a condenser, a photointerruptor or the like; and a connectingelement such as a connector, a switch, a printed circuit board, aresin-laminated semiconductor substrate, a lead frame or the like.Preferably, these members are of the shape of a plate with or without aneven surface.

A plating solution contained in the plating bath in accordance with thepresent invention is not particularly limited, and may be of anycomposition. Preferably, the plating solution does not contain lead.Further, preferably it does not contain cyanide. An example of a platingsolution employed is a plating solution constituting a known acidic bathor a complex salt bath to which a complexing agent is added.

Specific examples are inorganic acids including sulfuric acid, hydrogenhalide, sulfamic acid, phosphoric acid and pyrophosphoric acid;carboxylic acids including acetic acid, succinic acid, glycolic acid andcitric acid; hydroxycarboxylic acids and amino acids, dervatives thereofand the like; amine carboxylic acids including ethylenediamintetraaceticacid and iminodiacetic acid; hydroxyalkane bisphosphonic acids including1-hydroxyethane-1,1-bisphosphonic acid; aliphatic and aromatic organicsulfonic acids including methane sulfonic acid and 2-hydroxypropanesulfonic acid; mercapto compounds including thioglycolic acid andacetylcysteine; and sulfur-containing compounds including thiourea andtrimethylthiourea.

Examples of a source of metallic ions constituting the plating bath aresalts and complexes of tin, copper, silver, bismuth, indium, antimony,zinc, gold, palladium and the like; and complexes of the above-mentionedacids and complexing agents. Among these, salts and complexes of tin orcopper are preferable.

The concentration of a metal, concentration of a free acid, pH,temperature, and kind and concentration of an additive, of a platingsolution may be properly adjusted. The concentration of a metal of aplating solution, for example, may be 1 to 100 g/l.

The additive of a plating solution may be properly selected inaccordance with the kind of the plating solution, and may be a nonionic,cationic, anionic or ampholytic surfactant, for example. The surfactantmay be any known surfactant.

Also, any known substance used as a smoothing agent or brightener may beadded. Examples are high molecular compounds including gelatin, peptone,polyethylene glycol and polyacrylamide; derivatives of sulfanilic acidincluding N-butylidenesulfanilic acid and aldol, and salts thereof;triazoles including benzotriazole and 4-hydroxybenzotriazole, andderivatives thereof; benzothiazoles including benzothiazole and2-methylbenzothiazole, and derivatives thereof; imines; triazinesincluding 2,4-diamino-6-[2′-methylimidazolyl(1′)ethyl-1,3,5-triazine and2,4-diamino-6-[2′-ethylimidazolyl(1′)ethyl-1,3,5-triazine; esters ofaromatic hydroxycarboxylic acid including o-(or m- or p-)methyl benzoateand phenyl salicylate; aldehydes including formaldehyde andacetaldehyde; diketones including glyoxal and diacetyl; anilinederivatives including aniline and o-(or m- or p-)toluidine;mercaptocarboxylic acids including thioglycolic acid andmercaptosuccinic acid; amine alcohols including triethanolamine anddiethanolamine; aliphatic primary and secondary amines; compounds havingconjugated double bond including acrylic acid and methacrylic acid.Further, such an antioxidant as resorcinol, pyrocatechol, hydroquinone,phloroglucinol, pyrogallol, hydrazine or ascorbic acid may be added.

The plating bath has a pair of cathode and anode, or a plurality ofcathodes and a plurality of anodes therein. The number, size, shape andthe like of the cathode and anode are not particularly limited and maybe properly adjusted. Preferably, the plating bath has a plurality ofcathodes and a plurality of anodes therein. Suitably, the anode is ofthe shape of a plate made of tin, SnCu or the like, for example.Suitably, the cathode is made of a material in the form of aplate-shaped or frame-shaped rack which allows electric current to floweffectively through the object to be plated so as to serve as a cathodetogether with the object to be plated. The cathode needs to be soconstituted as to be capable of varying current.

The object to be plated is treated at a first current density for apredetermined period in the plating bath having the cathode and anode asmentioned above to plate. The predetermined period here is notspecifically limited, and may be properly adjusted in accordance withthe thickness of a plating film to be obtained, current density or thelike. The predetermined period may be about 5 to 10 minutes, andpreferably about 7 to 8 minutes, for example. The first current densitymay be properly adjusted in accordance with the thickness of a platingfilm to be obtained, period for plating or the like. The first currentdensity may be about 0.1 to 20 A/dm², and preferably about 1 to 10A/dm², for example. Specifically, where a plating film of a thickness ofabout 8 to 20 μm is intended, the object to be plated may be treated atthe first current density of about 1 to 5 A/dm², and preferably about1.5 to 4.5 A/dm², for a period of about 7 to 8 minutes.

In the present invention, any type of plating method may be employed.For example, a continuous line type, carrier over type plating method(in which a plating bath is divided into a plurality of sections, and anobject to be plated is picked upward from a section and transferred intoan adjacent section, and this operation is repeated) and the like may bementioned. Among these, the continuous line type plating method issuitably used from the standpoint of efficiency and production costs.Specifically, it is preferably used as follows. That is, a plurality ofcathodes (hereafter, referred to as racks) each in the form of a rack,for example, are supplied continuously at predetermined intervals into aplating bath where a plurality pair of anodes are provided atpredetermined intervals. Each rack on which a plurality of objects to beplated are carried longitudinally and/or transversely is immersed at apredetermined region (cathode region) between a pair of anodes whileelectric power is supplied to plate at the region for a predeterminedperiod; and then transferred to an adjacent cathode region to againplate. Where the carrier over type plating method is employed, it issuitable to station a rack in a bath while supplying electric power fora predetermined period, in place of transferring the rack at apredetermined rate. The objects to be plated may be arranged such thatits principal surface on which a plating film is to be formed ishorizontal, perpendicular or oblique to a principal surface of an anode.Preferably, the objects to be plated are carried on the rack so as tomake an tilt angle of about 5 to 85°, and preferably 45 to 85°, withrespect to the anode. The tilt angle here means a tilt angle madebetween the principal surface of the object to be plated on which theplating film is to be formed and the principal surface of the anode sothat the principal surface of the object to be plated is horizontal,perpendicular or oblique to the principal surface of the anode (seeFIGS. 5(a), 6(a) and (b)). Where a plurality of objects to be platedwhich are disposed close to one another are opposed to the anode, it ispreferable that, in a projection area of the objects to be plated to theanode, an overlap area of two adjacent objects to be plated makes up athird or less of all the projection area of one object to be plated.

Subsequently, the object to be plated is maintained at a second currentdensity lower than the first current density. The second current densityis not particularly limited if it is lower than the first currentdensity, and may be about 0.5 to 4.5 A/dm², and preferably about 1.5 to2.5 A/dm², for example. The maintaining period here may be properlyadjusted in accordance with the thickness and material of a platingfilm, current density or the like, and may be about 5 to 10 minutes, andpreferably about 7 to 8 minutes, for example. Where the method forforming a plating film of the present invention is implemented by theabove-mentioned continuous type plating method, it is preferable totransfer the plurality of cathodes each of which carries a plurality ofobjects to be plated at a rate of about 0.5 to 5 cm/sec. and preferablyabout 1.0 to 3.5 cm/sec., in accordance with the length of a platingbath and to change the density of electric current flowing of thecathodes to the second density at a last cathode region, i.e., a regionwhere a rack having objects to be plated passes through between a lastpair of anodes in the plating bath. The application of the secondcurrent density allows a previously obtained plating film to have adense and even surface while thickening it slightly.

Also, in accordance with the present invention, it is preferable toneutralize after the plating film is applied, in place of maintainingthe plating film at the second current density lower than the firstcurrent density. Preferably, this neutralization is carried out afterthe plating solution is washed with water after the application of theplating film. The neutralization here can be carried out by immersingthe object to be plated into an organophosphorus treating agent,specifically a trisodium phosphate solution, for example. Theconcentration of the organophosphorus treating agent here may be about 5to 10 wt %, and temperature thereof may be about 25 to 65° C., andpreferably about 55 to 65° C. The period for immersion may be about 10to 60 seconds, and preferably about 20 to 40 seconds.

The neutralization may be carried out in the same manner as mentionedabove after the object to be plated is maintained at the second currentdensity, and preferably after the plating solution is rinsed after theobject to be plated is maintained at the second current density.

Also, in the method for forming a plating film in accordance with thepresent invention, optionally, the above-mention process may follow orbe followed by such treating processes known in the art as formation ofan underlying film, degreasing, rinsing, etching, activation using acid,thermal rinsing, draining, drying and further formation of a film. Theformation of an underlying film, for example, means formation of a filmof a nickel, nickel alloy, copper, copper alloy or the like film on theobject to be plated by electroplating or non-electrolytic plating beforethe formation of a plating film as mentioned above. The furtherformation of a film means formation of a single-layered film or alaminate films of gold, silver, palladium, bismuth, antimony, indium orthe like, or an alloy of two or more of these metals.

The electronic component in accordance with the present invention has anon-glossy plating film formed on a surface of a conductive material asmentioned above. The glossiness here may be adjusted properly inaccordance with the size and smoothness of each of particles on thesurface of the plating film; and the brightener used in the platingsolution or the like, and may be about 0.1 to 0.4, preferably about 0.1to 0.35, and more preferably about 0.2 to 0.33 (measured by, forexample, a densitometer GAM Model RD-144 manufactured by ElectroplatingEngineers Co., Japan). Also, the plating film is formed preferably of analloy containing two metals of Sn and Cu, more preferably of an alloycontaining Sn and 90 wt % or less of Cu, and still more preferably of analloy containing Sn and 1 to 5 wt % of Cu. Further, preferably theplating film is about 8 to 20 μm thick, and more preferably there areparticles of Cu or an Sn—Cu alloy with a 1 μm or less diameter disperseduniformly in the plating film.

These and other objects of the present application will become morereadily apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

Now, there will now be explained in detail embodiments of a method forforming a plating film and an electronic component having a plating filmformed thereon in accordance with the present invention based on thedrawings. It should be understood that the present invention is notlimited to the embodiment.

Embodiment 1

First, a photoreceptor and a light emitting element are respectivelydie-bonded by an Ag paste to lead frames of Cu, a Cu alloy or a Fe alloyas a base material. The photoreceptor and the light emitting element arerespectively wire-bonded to the adjacent lead frames by gold wires.

Next, the light emitting element is precoated with a silicone resin forreduction of stress, and then the lead frames having the elementsdie-bonded thereto are spot-welded to position the photoreceptor and thelight emitting element in an opposing relation. Then, primary transfermolding is carried out using a light transmitting epoxy resin to form aninner package such that an optical path is formed from the lightemitting element to the photoreceptor. Subsequently, excess resin thathas been leaked outside is deburred, as indicated in step 1-1 of FIG. 1and secondary transfer molding is carried out using a light shieldingresin to form an outer package such that optical signals can betransmitted from the light emitting element to the photoreceptor withoutentrance of disturbance light nor leakage of light from inside.

Then, the package is subjected to degreasing (step 1-2), rinsing (step1-3), etching (step 1-4), again rinsing (step 1-5), activation (step1-6) using acid and further again rinsing (step 1-7) to clean thepackage, and then a plating film for exterior is formed on the leadframes of a packaged semiconductor device (step 1-8).

Firstly, an alkanolsulfonic acid plating bath which contains a SnCuplating solution is prepared. The plating bath is constituted mainly bymethanesulfonic acid, adjusted the concentrations of Sn and Cu to haveCu at 1 to 5 wt % in the plating bath and added an additive FCM-10(manufactured by Yuken Industries, Co., Japan) of about 30 to 50 ml/l(preferably about 35 to 45 ml/l) for example as well as a chelatingagent (manufactured by Yuken Industries, Co.) for inhibiting anodicsubstitution of divalent Cu after the preparation of the platingsolution. Then, a semiconductor device 12 having a lead frame 13 mountedon a cathode rack 11 is immersed into the obtained plating bath betweena pair of anodes 10 for about 7 to 8 minutes, as shown in FIG. 2, whileapplying a current density of about 4.0 to 5.0 A/dm² to the cathode rack111 during the immersion. Thus, a non-glossy plating film of a thicknessof about 15 μm is formed on a surface of the lead frame.

Next, rinsing (step 1-9) and then neutralization (step 1-10) are carriedout. As the neutralizing solution here, there is used a trisodiumphosphate (Na₃PO₄) solution at a concentration of about 10 wt % and atemperature of about 55 to 65° C., into which the lead frame is immersedfor 20 to 40 minutes.

Thereafter, thermal rinsing (step 1-11), draining (step 1-12) and drying(step 1-13) are carried out to obtain a semiconductor device where theplating film is formed on the lead frame.

Thereafter, thermal rinsing, draining and drying are carried out toobtain a semiconductor device where the plating film is formed on thelead frame.

In the plating film provided on the lead frame by such a method, theunevenness at its surface is alleviated and small particles of Cu or anSn—Cu alloy with a diameter of 1 μm or less are uniformly dispersed inthe non-glossy, single-layered SnCu plating film, as shown in FIG. 3.

For comparison, another plating film was formed by preparing analkanolsulfonic acid plating bath which contains a SnCu platingsolution; adjusting the concentrations of Sn and Cu to have Cu at 1 to 5wt % in the plating bath; properly adding an additive different fromFCM-10 as well as a chelating agent; immersing a semiconductor deviceinto the plating solution at a current density of about 6.0 to 7.0 A/dm²for about 7 to 8 minutes; and carrying out neutralization usingtrisodium phosphate at a temperature of about 20 to 25° C. Thus, aplating film is obtained about 8 to 20 μm thick.

In the plating film provided by such a method, the unevenness developedon its surface and particles of Cu or an Sn—Cu alloy with a maximumdiameter of about 4 μm were formed, as shown in FIG. 4.

Also, solderability was evaluated with respect to semiconductors whereplating films are formed on lead frames by the same method as that usedfor the above-mentioned plating film.

For the evaluation of solderability, a sample of a substrate-mountingtype (component which is solderable by inserting a lead frame terminalinto a predetermined hole in a printed substrate) and a sample of asurface-mounting type (component which has a terminal bent horizontallyto a surface of a printed substrate so that the component can besoldered to a wiring on the printed surface by reflow soldering or thelike) were pretreated by allowing both samples

-   -   to strand at 100° C. and at 100% RH for 8 hours using an        acceleration steam aging apparatus, or    -   to subject at 105° C. and at 100% RH for 8 hours on PCT        (Pressure Cooker Test) and then allowing them to stand at a room        temperature for two hours or more, as described in attachment A        of JIS0050. After the pretreatment, an equilibrium method EIAJ        ET-7401 and a quick heating method EIAJ ET-7404 were applied to        the substrate-mounting type sample and surface-mounting type        sample, respectively.

Also, Sn_(2.5)Ag_(0.5)Cu_(1.0)Bi was used as solder and the heatingtemperature of the solder was set to 245° C.

It was found that the zero cross time was 3 seconds or less.

Embodiment 2

A semiconductor device 22 having a lead frame 23, which is the same leadframe as that in Embodiment 1, is mounted on a cathode rack 21, as shownin FIG. 5(a), and immersed into a same plating bath 24, which is thesame plating bath as that in Embodiment 1 In the plating bath 24, thecathode rack 21 is immersed at a region between anodes 20 while electricpower is supplied to the cathode rack 21 to plate at the region for apredetermined period; and then transfered to an adjacent cathode regionto plate, as shown in FIG. 5(b).

Then, when the cathode rack 21 reached a last cathode region, thecurrent density to the cathode rack 21 is changed to about 2.5 A/dm² andthe cathode rack 21 is transfered at a constant rate for about 1 to 2minutes to obtain a non-glossy plating film of a thickness of about 15μm on the lead frame 23.

Also in the thus obtained plating film, small particles of an Sn—Cualloy with a diameter of 1 μm or less are uniformly dispersed to allowthe plating film to have a dense surface, as shown in FIG. 3.

Embodiment 3

Plating films are formed in the same manner as in Embodiments 1 and 2,except that semiconductor devices 32 having lead frames are mounted on acathode rack 31 such that a predetermined angle of, for example, 50° isformed between the semiconductor devices 32 and an anode 30, as shown inFIGS. 6(a) and (b). Also, in a projection of the lead frames 33 to theanode 30, an overlapping area of two adjacent lead frames makes up athird or less of all the projection area of one lead frame. This makesit possible to form, while considering mass production, a plating filmof a predetermined thickness owing to an electrodeposition effect thatallows plating to proceed also at a portion where adjacent lead framesappear to overlap.

In other words, generally, the lead frames, i.e., objects to be platedare mounted on the cathode rack substantially perpendicular to orsubstantially in parallel with the anode for plating the lead frames.Mounting the lead frames perpendicularly to the anode can secure massproduction but generally produces a tendency of the plating filmthickness to be thicker in a region nearer to the anode (region A or A″in FIG. 2) and thinner in a region more distant from the anode (regionA′ in FIG. 2). As the distance from region A to region A″ is increased,the unevenness of the plating film thickness will be increased. On theother hand, mounting the lead frames in parallel to the anode decreasesthe number to be treated of lead frames but can inhibit the unevennessof the plating film thickness.

In comparison, this embodiment is advantageous in that while theproductivity of the lead frames, i.e., plate-shaped objects to be platedis enhanced, there can be obtained a SnCu plating film of high qualitywhich is excellent in solderability even with minimized unevenness inthickness.

The present invention makes it possible to improve solderability of aplating film for conventional lead-free solder by a simple method, whichallows the productivity to further enhanced, resulting in a plating filmwith reduced production costs. Especially, where the object to be platedis plated at current densities in two levels, or where a plating film isapplied and then the object to be plated is immersed into a solutioncontaining 5 to 10 wt % of trisodium phosphate, a more uniform platingfilm can be obtained and thereby solderability of the plating film forsolder can be promoted.

Especially, where the object to be plated is opposed to the anode suchthat a tilt angle of 5 to 85° is made therebetween for plating, or wherea plurality of objects to be plated which are disposed close to oneanother are opposed to the anode such that in a projection of theobjects to be plated to the anode, an overlap area of two adjacentobjects to be plated makes up a third or less of all the projection areaof one object to be plated, the unevenness of a plating film thicknessdepending on regions of the plating film can be inhibited, which makesit possible to form a plating film that satisfies solderability to agreater extent.

Also, as a result of a research on lead-free solder especially using anSnCu plating film, it has been found that particles of Cu or an Sn—Cualloy with a 2 μm or less diameter are non-uniformly present (abnormallydeposited) in a non-glossy SnCu plating film which is poor insolderability to damage solderability, but present invention enablesparticles of Cu or an Sn—Cu alloy which are of a diameter of about 1 μmor less to be uniformly dispersed and deposited so that the resultingSnCu plating film can provide remarkably improved solderability.Further, it has been found that a plating film having a thinner region(<8 μm) suffers from inconsistencies in solderability, but the presentinvention makes it possible to obtain a plating film of an uniformthickness and therefore to provide further improved solderability.Moreover, it has been found that a plating film with a less roughenedand more dense surface tends to provide favorable solderability, and thepresent invention makes it possible to obtain a plating film having adense surface and therefore to provide further improved solderability.

1. An electronic component having a plating film formed thereon by amethod comprising: applying a plating film onto an object to be platedat a first current density for a predetermined period in a plating bathhaving a cathode capable of varying current and an anode and; andmaintaining the object to be plated at a second current density lowerthan the first current density; and neutralizing the object to be platedwith a trisodium phosphate solution.
 2. An electronic component of claim1, wherein the plating film is made of an Sn—Cu alloy containing 1 to 5wt % of Cu.
 3. An electronic component of claim 1, wherein the platingfilm is a non-glossy plating film of about 8 to 20 μm thick and hasparticles of an alloy with a 1 μm or less diameter dispersed uniformlytherein.
 4. An electronic component having a plating film formed thereonby a method comprising: applying a plating film of an alloy comprisingSn and Cu onto an object to be plated at a first current density for apredetermined period in a plating bath having a cathode and an anode;and treating for neutralizing the object to be plated with a solutioncontaining 5 to 10% wt of trisodium phosphate.
 5. An electroniccomponent of claim 4, wherein the plating film is made of an Sn—Cu alloycontaining 1 to 5 wt % of Cu.
 6. An electronic component of claim 4,wherein the plating film is a non-glossy plating film of about 8 to 20μm thick and has particles of an alloy with a 1 μm or less diameterdispersed uniformly therein.